1. Field of the Invention
The present invention is directed to a process for the hydrogenation of polyunsaturated oils to produce products having a high monounsaturated fatty acid (monoene) content, wherein the process utilizes a reusable copper-chromium containing catalyst to reduce the degree of unsaturation in the oil. The resulting product has improved stability and a longer shelf life.
2. Related Art
Polyunsaturated oils are hydrogenated to reduce the degree of unsaturation in the oil, prior to subsequent processing to obtain secondary products, such as food grade oils, additives, lubricants and the like.
Plant oils are useful as components for many nutritional products such as nutraceuticals, and vegetable oils. However, before the oils can be used commercially, particularly for nutritional use, the components responsible for offensive odors, poor taste and poor stability must be removed. The major component contributing to these unfavorable characteristics is the content of polyunsaturated oils, particularly linolenate, present in the oil. The content of linolenate (C18:3) in the oil, therefore, is reduced to the more desirable component, monounsaturated fatty acid (C18:1).
Reduction of the double bond content in polyunsaturated oils is traditionally carried out by partial hydrogenation, catalyzed by a transition metal catalyst. Various transition metal catalyst, such as nickel, copper, cobalt, palladium and platinum have been used as hydrogenation catalyst. However, copper-chromium combination catalysts (i.e., copper chromite catalyst) have been found to be more selective for production of the monoene. The hydrogenation of the polyunsaturated oils with copper chromite stops with the monoene, with little to no production of the saturated fatty acid in the oil. Popescu et al., JAOCS 46(2):97-99 (1969); Koritala et al., JAOCS 43(2):86-89 (1966).
In a later study, Koritala hydrogenated 300 mL samples of soybean oil at moderate pressures (i.e., 30 psig), in the presence of 0.1 and 0.5% copper-chromium catalyst, at temperatures of 170 and 200xc2x0 C. Koritala, et al., JAOCS 43(9):556-558 (1966). For those catalysts tested, the average monoene conversion at 170xc2x0 C. was 37%, and that for 200xc2x0 C. was 34%. Id. The results, therefore, show that the conversion and selectivity are better at lower temperatures. Id. at 557.
The above studies show that copper-chromium containing catalysts are suitable for hydrogenation of polyunsaturated oils. However, the percent conversion to monoene under the above-described reaction conditions is low (i.e., 34-37%). Another drawback to the above-described processes is that transition metal hydrogenation catalyst are susceptible to poisoning during the reaction process, which decreases the selectivity, yield and stability of the hydrogenated product. For example, Moulton et al. used a heat activated copper-chromium-barium catalyst to hydrogenate soybean oil for five consecutive hydrogenations, without the addition of additional catalyst to the process. Moulton et al. JAOCS 43(12):662-666 (1969). A progressive drop in the selectivity for the conversion for each hydrogenation was observed. Id. at 664. Additionally, for the second hydrogenation, the reaction time to obtain a product having an Iodine Value (IV) of 120 was about twice that of the first hydrogenation. Id.
Generally, once poisoned, the catalyst cannot be reused, or it must be reactivated, which is time consuming and costly. Typical catalyst poisons are chlorine (see Heldal et al JAOCS 59(9):396-398 (1982)), sulfur and free fatty acids (see U.S. Pat. No. 5,250,713) and water vapor resulting from the reduction of the catalyst, and oxidation products formed during hydrogenation (see Johansson et al. JAOCS 56(12):981-986 (1979)). The oxidation products from fatty acids also contribute to the unfavorable taste and odor of the soybean oil, and must be removed prior to processing into food grade oils. Additionally, trace amounts of copper, in the hydrogenated oil, further contribute to the unfavorable taste, odor and stability of soybean oil, and must be removed.
In view of the above, it would be desirable to utilize a catalyst that is resistant to poisoning and can be reused after an initial hydrogenation, without a need for regeneration or replacement with fresh catalyst. It would further be desirable to obtain from a polyunsaturated oil hydrogenation process, a monounsaturated fatty acid percent conversion of greater than about 60%, and preferably greater than about 70%, and more preferably greater than about 80%.
A method for improving the resistance of copper-chromium catalysts to poisoning during hydrogenation, and additionally improving the percent conversion of the hydrogenated polyunsaturated oil to the monounsaturated form has been discovered by applicants, and is the focus of the invention described below.
The invention is directed to a process for hydrogenating a polyunsaturated oil, comprising:
(a) reacting said polyunsaturated oil with hydrogen in the presence of a copper-chromium containing catalyst, at a temperature of from about 200-300xc2x0 C. and a pressure of from about 300-500 psig, for about 3-8 hours;
(b) removing said catalyst from a product obtained in (a); and
(c) recycling said removed catalyst obtained from (b) to a subsequent hydrogenation reaction, wherein said hydrogenation reaction is carried out as described in (a).
In a first embodiment of the invention, applicants have discovered a process for hydrogenating a polyunsaturated oil, wherein the catalyst is reusable. The process of this embodiment comprises:
(a) reacting said polyunsaturated oil with hydrogen in the presence of a copper-chromium containing catalyst, at a temperature of from about 200-300xc2x0 C. and a pressure of from about 300-500 psig, for about 3-8 hours;
(b) removing said catalyst from a product obtained in (a); and
(c) recycling said removed catalyst obtained from (b) to a subsequent hydrogenation reaction, wherein said hydrogenation reaction is carried out as described in (a).
Various polyunsaturated oils from animal and plant sources may be used in the hydrogenation process. Polyunsaturated oils derived from plant sources are preferred. Preferred polyunsaturated oils, obtained from plant sources, may be selected from babassu, coconut, canola, corn, cotton seed, linseed, oiticica, olive, peanut, perilla, rapeseed, safflower, sesame, soybean, sunflower seed, tung and wheat-germ. Of these polyunsaturated oils, canola, corn and soybean are particularly preferred. Soybean oil is most particularly preferred. The plant source from which these oils are obtained, may also be a genetically engineered or modified plant, which may have a fatty acid compositional content that is different than that of a non-genetically engineered or non-modified plant of the same species.
In a second embodiment of the invention, the once recycled catalyst may be reused in subsequent hydrogenation reactions as described in (a) above, without further treatment. Under the conditions of the present invention, the number of times the catalyst may be reused will depend on how long it takes the catalyst to lose a significant amount of its activity and selectivity for conversion to the monoenes. In this instance, the term xe2x80x9csignificant amountxe2x80x9d is defined as a point during the re-use of the catalyst, where a loss of catalytic activity and/or selectivity begins to produce a less than desirable hydrogenated product.
Typically, for conversion to the monoenes, the catalyst may be reused until a loss of at least about 50% of catalytic activity, catalytic selectivity or both, is obtained. The number of times the catalyst may be reused is from about 1-100 times and preferably 4-20 times. Factors that may affect the activity and/or selectivity of the catalyst include, but are not limited to, impurities present in the feedstock oil or incomplete removal of the catalyst from a previous re-use cycle. Generally, the number of times the catalyst may be reused is a function of the ability of the catalyst to continually produce a hydrogenated product, having desired characteristics.
In a third embodiment of the invention, applicants have discovered that the above-described reaction produces a hydrogenated oil in which percent conversion of polyunsaturated fatty acids to the monounsaturated fatty acids is from about 60% to about 90%. A more preferred percent conversion is from about 70% to about 90%, and most preferably a conversion of from about 80% to about 90% may be obtained.
The hydrogenated oil product may further be separated into liquid and solid fractions using fractionation processes known to those of skill in the art. Uses for the liquid fraction include, but are not limited to, vegetable oils, pharmaceuticals and lubricants. Uses for the solid fraction include, but are not limited to, food grade greases, lubricants and components for candles.
The hydrogenated oil product may also be converted to, among other things, diesters of diols, which are useful as lubricants. Additionally, they may be subjected to hydroformylation, to form fatty acid alcohols, aldehydes, imines, amines and the like. Thus, a fourth embodiment of the invention is directed to using the monoenes obtained from the above-described process, as reactants to form other useful products.
The monoene product is generally a triglyceride containing monounsaturated fatty acids. Where the monoene product is used to produce vegetable oils, ultimately the cis form of the monounsaturated fatty acid is the desired reaction product, due to its lower melting point and generally liquid state. However, isomerization to the trans form of the monounsaturated fatty acids occurs during the hydrogenation. The trans form of the monounsaturated fatty acids has a higher melting point, is generally a solid at room temperature, and is less desirable for vegetable oils. Some of the trans form of the monounsaturated fatty acid may be separated from the cis isomer by fractionation of the hydrogenated product. Thus, a fifth embodiment of the invention comprises enrichment of the cis form of the obtained monoene.
The copper-chromium catalysts used to catalyze the hydrogenation of the oil, in accord with the aspects of the invention, are generally compositions having mixtures of CuO and Cr2O3 or CuO and CuCr2O4, in various empirical ratios, and also may optionally include other metal oxides which act as promoters, such as BaO. Copper-chromium catalysts are commercially available in a wide range of empirical ratios of CuO:Cr2O3, such as, for example, 51% CuO and 47% Cr2O3; or CuO:Cr2O3:BaO, such as, for example, 40% CuO, 47% Cr2O3 and 10% BaO. Generally, any of the commercially available copper-chromium catalysts (with or without a promoter metal oxide) have suitable composition ratios and are effective for hydrogenation of the oils, in accord with the invention.
The catalyst may be prepared from the decomposition Cu(OH)NH4CrO4 in air, at 350-500xc2x0 C., for about 2 hours. X-ray diffraction analysis shows that during the air heating, the product catalyst has two separate crystalline phases (i.e., CuO and CuCr2O4). In some of the commercial copper-chromium catalysts, approximately 63 wt. % of the material constitutes the CuCr2O4 crystalline phase. Other known methods of preparing copper-chromium hydrogenation catalysts may be employed to make suitable catalysts for use in the aspects of the invention.
The resulting catalysts may optionally be used in a powder or granule form, or may be coated on inert support materials and supported for use in accord with the invention. For example, the catalyst may be incorporated within a fixed bed catalyst system.
The following examples are presented to further describe the invention and are not intended to be limiting in any way.